1. Field of the Invention
This invention relates to an optical disc apparatus for reading and reproducing data recorded on an optical disc such as a CD or a DVD.
2. Description of the Related Art
Conventionally, in an optical disc apparatus for reading data recorded on an optical disc such as a CD or a DVD, focus control for matching a focusing position of applied laser light with a record surface of the optical disc has been performed. This focus control is control in which an objective lens arranged between the optical disc and an LD which is a light source is moved in a direction of approaching to and receding from the optical disc. The LD, the objective lens, an actuator for moving this objective lens in the direction of approaching to and receding from the optical disc, etc. are disposed in a pickup head. A signal inputted to the actuator in the focus control is a signal in which a focus servo signal generated based on a focus error signal (hereinafter called FE signal) indicating the amount of deviation of the record surface of the optical disc from the focusing position of the applied laser light is superimposed on a focus bias as is publicly known.
This focus bias is adjusted to a voltage in which amplitude of an RF signal which is a reading signal of data recorded on the optical disc is maximized. In other words, the optical disc apparatus is provided with a configuration for adjusting the focus bias so as to maximize the amplitude of the RF signal. This focus bias adjustment includes, for example, a method for stepwise changing a focus bias and sampling an RF signal and adjusting the focus bias to a voltage in which amplitude of the RF signal is maximized (see JP-A-2002-312959). Also, there are a method for adjusting a focus bias so as to minimize an average value of an error rate or the amount of jitter of an RF signal (see JP-A-11-120612), and a method for making adjustment based on two focus bias voltages in which the same jitter value is obtained in regions of both sides beyond a dead zone region in jitter value characteristics indicating a relation between the jitter value and the focus bias voltage (see JP-A-7-44882).
However, in the conventional optical disc apparatus, when a focus bias is adjusted in the inner circumference side of this optical disc in the case of setting the optical disc in a body, the focus bias adjusted herein is used during reading of data from this optical disc. In other words, the conventional optical disc apparatus had a configuration in which the focus bias is first adjusted only once. On the other hand, the optimum voltage value of the focus bias varies in a radial direction of the optical disc because of warpage etc. of the optical disc. As a result of this, the conventional optical disc apparatus first adjusts the focus bias in the inner circumference side only once as described above, so that there was a problem that data recorded on the outer circumference side of the optical disc is read with the focus bias deviating from the optimum voltage and the data cannot be correctly read from the optical disc.